1. Field of the Invention
The present invention generally relates to a method of depositing a film by atomic layer deposition (ALD); particularly to a method of depositing a film by ALD with pulse-time-modulated plasma.
2. Description of the Related Art
Plasma Enhanced Atomic Layer Deposition (PE-ALD) is one of the plasma deposition processes. This method forms a film by repeating a process of adsorbing a precursor on a wafer and a process of treating the adsorbed precursor with a reactant gas plasma wherein the reactant gas plasma reacts the precursor adsorbed on the surface, thereby forming an atomic sub-layer on the wafer in each cycle of the repeating processes. To deposit a thick film, it requires a cyclic deposition of at least several hundred times, taking a long time to complete the deposition process. In the above, since plasma treatment is repeated at least several hundred times, surface potential on the wafer increases as a result of accumulating floating potential on the wafer, leading to the following risks: wafer slip, wafer sticking on a susceptor, and destruction of gate elect loads in a patterned wafer.
However, the above surface potential problems have not been recognized because PE-ALD predominantly targets a thin film and thus, it is believed that surface potential does not increase significantly on the wafer. Further, because PE-ALD is based on an equalization process of precursor adsorption and planar plasma treatment by a reactant gas, in-plane uniformity of a film is believed to be good. Thus, no solutions to the aforementioned problems have been provided. However, the present inventor expects that the problems will become more significant in the future because there is a demand for finer, more complex layered structures and patterns, and a combination with other layers and other treatments, wherein surface potential and in-plane uniformity of a film by PE-ALD may affect characteristics of a final product.
Technology to remove surface potential from a wafer is known in CVD, wherein amplitude-modulated RF power is applied, as a post-film formation process, after completion of film formation on the wafer (U.S. Patent Publication No. 2009/0136683). However, this technology is not suitable for PE-ALD because PE-ALD is a highly cyclic process, and surface potential accumulates at each cycle. Use of RF power having a pulse-time-modulation of 10 to 100 μsec for etching is known (Japanese Patent Laid-open No. H08-181135). However, such short pulse-time-modulation is used to alternately emit positive ions and negative ions to a wafer, thereby inhibiting accumulation of electrons and increasing etching speed, and this technology is unrelated to PE-ALD.
Any discussion of problems and solutions involved in the related art has been included in this disclosure solely for the purposes of providing a context for the present invention, and it should not be taken as an admission that any or all of the discussion were known at the time the invention was made.